1. Field of the Invention
The present invention relates to improvements in controlling the energization of electroluminescent lamps. More specifically, this invention is directed to an improved driver circuit for an electroluminescent (EL) display and particularly to an EL lamp driver which emulates a constant current source. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
Circuits for energizing, i.e., driving, EL lamps are well known in the art. For examples of such prior art lamp drivers, reference may be had to the present inventor's co-pending application Ser. No. 793,874 and U.S. Pat. No. 5,066,895. U.S. Pat. No. 5,066,895 also provides a discussion of the construction and utility of EL lamps.
An EL lamp is a light-emitting device which appears to the circuit supplying energy thereto as a capactive load. As an EL lamp ages, its efficiency changes very little, provided the aging is intrinsic and not the result of moisture ingress. Thus, the power consumed by an EL lamp per lumen of light output is relatively constant. Accordingly, if an EL light could be operated from a constant current source, its brightness would remain relatively constant with time even though the electrical characteristics, particularly the capacitance, of the lamp would change to some degree.
Prior art EL lamp drivers, pursuant to an effort to maintain constant brightness over the service life of an EL lamp, and faced with the lamp operational characteristic of a decrease in capacitance with age, have had a tendency to be complex devices. Circuit complexity, which is typically manifested in the use of a significant number of components, is the antithesis of volumetric efficiency and also necessarily results in diminished reliability. With respect to volumetric efficiency, as circuit complexity increases, the ability to fabricate the lamp driver as an integrated circuit is significantly reduced. It is also to be noted that driver circuit cost increases with complexity and many prior art EL lamp drivers have employed comparative expensive transformers.